This invention pertains generally to telephony, and more particularly to packet-switched telephony.
Packet-switched telephony is an alternative to traditional circuit-switched telephony. In circuit-switched telephony, a circuit is dedicated to each active telephone call. In packet-switched telephony, the audio signal is partitioned into data xe2x80x9cpacketsxe2x80x9d that are individually routed over a data network, sharing the network""s pathways with packets from many other sources. Before a packetized audio segment can be played out, its packets must be received and re-assembled.
Packet telephony has several advantages when compared with circuit-switched telephony. Packet telephony uses network resources efficiently, since packets need not be sent when no audio signal is present on the line. Packet telephony also often employs audio codecs (coder/decoder pairs) in order to compress the amount of data that must pass over the network. And with at least some packet telephony systems, users may avoid long distance charges that would be incurred with a circuit-switched connection.
Packet telephony also has disadvantages. Whereas a circuit-switched connectionxe2x80x94once establishedxe2x80x94usually provides fairly consistent connection quality over the length of the call, a packet telephony connection may deliver widely-varying connection quality. Quality variations generally mirror the instantaneous packet delivery performance of the packet-switched connection, which in turn depends on the level of competing traffic. As traffic increases, packets may be delayed, increasing the latency of the connection. An increase in the interarrival jitter, which is an expression of the variation in packet transit times, also increases latency. In some cases, the network or receiver may throw away packets that have been delayed too long, causing dropouts in the reconstructed speech.
A typical packet-switched telephone connection is illustrated in FIG. 1. A calling party 20 connects over a local circuit-switched network 24 to a packet voice gateway 28. After connection, calling party 20 signals the number of a desired party to gateway 28. Gateway 28 attempts to locate a second gateway 30 that is local to the desired party. If such a gateway is found, a packet-switched connection is established with gateway 30 and the desired party""s number is communicated to gateway 30. Gateway 30 connects to the desired party 22 through a second local circuit-switched network 26, and the connection is complete.
A typical gateway 28 is illustrated in FIG. 2. The gateway communicates with a circuit-switched network through a front end 54, which typically terminates one or more TDM (time-division multiplexed) digital PCM (pulse-code modulated) circuit-switched telephony signals, such as those following the well-known T1 or E1 standards. On the other end of gateway 28, a packet data transceiver 58 communicates with a packet-switched network, e.g., using IP (Internet Protocol) or ATM (Asynchronous Transfer Mode) packets. In between front end 54 and transceiver 58, one or more signal processors 56 perform audio processing functions such as voice activity detection, DTMF detection, and audio coding and decoding on the audio data streams. A host processor 52 controls and coordinates the operation of gateway 28.
It is recognized herein that the uncertain quality of service associated with packet telephony is a major detriment to its widespread acceptance. The present invention attempts to provide a more uniform quality of service by providing a mechanism for re-routing calls from a packet-switched connection to a circuit-switched connection, and preferably, from a circuit-switched connection to a packet-switched connection. In other words, if the quality of a packet-switched connection is poor, the present invention enables a call to be switched to a circuit-switched connection transparently and with no apparent loss of connection. Preferably, if conditions later warrant re-establishing the packet-switched connection, this can also be accomplished.
In one aspect of the invention, a packet voice switch is disclosed. The switch comprises a telephony interface and a packet data interface, and coding/decoding means for converting data from circuit-switched to packet-switched formats (and vice-versa) as it passes between the interfaces. The switch further comprises call switching means that routes calls between the telephony interface and the coding/decoding means. The switch also comprises network switching means (preferably implemented on a host processor) for selecting call routingxe2x80x94the network switching means can route calls received at the telephony interface either back out through the telephony interface over another circuit, or through the coding/decoding means to the packet data interface.
In a second aspect of the invention, a method of operating a telephony switch connected to both a circuit-switched network and a packet-switched network is disclosed. The method comprises calculating the estimated quality of a packet-switched connection for a given call, and selecting, based on that quality, either a packet-switched connection or a circuit-switched connection for that call. The method further comprises connecting the call over the selected network.
In another aspect of the invention, a method of operating a packet voice switch connected to both a circuit-switched network and a packet-switched network is disclosed. The method comprises the steps of receiving an original call request over the circuit-switched network and routing the call request over the packet-switched network to a remote packet voice switch for call connection to a called party. The method further comprises responding to a network change request by rerouting the call over a long-distance circuit-switched network to the same remote packet voice switch.
In yet another aspect of the invention, a second method of operating a packet voice switch connected to both a circuit-switched network and a packet-switched network is disclosed. The method comprises the steps of receiving and acknowledging network switch requests from a second packet voice switch. The method further comprises matching an incoming circuit-switched connection from the second packet voice switch with an existing packet-switched connection from the same second switch, and then switching call voice data from the packet-switched to the circuit-switched connection.